Low noise electron gun



Oct. 26, 1965 w. A. HARMAN LOW NOISE ELECTRON GUN Filed July 22, 1963MINI; U

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WARD A. HARMAN EW/z, M

ATTORNEY United States Patent Office 3,214,632 Patented Oct. 26, 19653,214,632 LOW NOISE ELECTRON GUN Ward A. Harman, Los Altos Hills,Calif., assignor to General Electric Company, a corporation of New YorkFiled July 22, 1963, Ser. No. 296,632 7 Claims. (Cl. 315--14) Thisinvention relates to an electron gun for producing and projecting a beamof electrons. The invention relates more particularly to an electron gunfor producing an electron beam of high electron current density withrelatively low noise. The invention is especially useful in a highpower, low noise traveling wave tube for providing extended dynamicrange.

In order to extend the usefulness of electron beam devices, considerableeffort has been expended to reduce the undesirable noise attendant theproduction and projection of the electron beam. Theories have beenadvanced in explanation of noise generators and noise reduction inelectron beams. See, for example, Noise in Electron Devices by L. D.Smullin and H. A. Haus, John Wiley and Sons, New York, 1959. However,the generation and the transformation of beam noise in the immediatevicinity of the cathode are not well explained by existing theories.Therefore, the production and projection of low noise electron beamsthus far remains, to a considerable extent, an empirical art.

Among the features now believed to be necessary to achieve low noise isthat of providing a region immediately following the region of electronemission wherein low axial drift velocity of the electrons ismaintained. Low noise electron guns including the foregoing feature havebeen constructed, for example, as described by M. R. Currie and D. C.Forster in New Mechanism of Noise Reduction in Electron Beams, Journalof Applied Physics, vol. 30, pages 94-103, January 1959. The electrongun therein described, while capable of producing a low noise beam,employs an annular cathode which, because of its limited effectiveelectron emission area, is not well adapted to the production of highpower beams. It is desirable to produce and project an electron beam ofhigh power with low noise.

It is therefore an object of the invention to provide an improvedelectron gun.

It is a specific object of the invention to produce a high powerelectron beam with low noise.

It is another object of the invention to provide a high perveanceelectron gun structure which provides an extended region of low electronvelocity immediately following the region of emission.

These and other objects of the invention are accomplished by an electrongun of the magnetron type having a large emissive area from which largeelectron currents can be drawn at relatively low emission densities andhaving special features for providing an extended low velocity driftregion immediately following the region of emission from the cathode forachieving minimum noise in the beam. The electrode shapes are alsodesigned to give smooth electron flow by minimizing the presence ofstrong perturbing electric or magnetic fields.

Briefly, the electron gun of the invention comprises a cathode ofcircular cross section having an emissive surface along a portionthereof for electron emission predominantly transverse of the axis ofthe cathode and of the beam path. This emissive portion of the cathodeis preferably tapered in the direction of the electron path forimparting an axial drift velocity to the emitted electrons.

An anode is positioned in spaced coaxial relationship to the cathode forextracting electrons therefrom, the anode having an internal boresurface at least coextensive with the emissive surface of the cathode,this internal bore preferably being formed with a taper.

The foregoing cathode-anode structure is immersed in a relatively strongmagnetic field which is substantially parallel to the longitudinal axisof the cathode with the result that the extracted electrons are causedto describe circumferentially directed paths around the cathode.

A plurality of accelerating electrodes are positioned forward of thecathode-anode structure along the path of the beam. The combined actionof the magnetic field and the electric fields of the acceleratingelectrodes, plus the influence of the cathode taper, causes theextracted electrons to move in spiral paths of decreasing radii towardthe acelerating electrodes whereby a dense hollow electron beam isformed and projected, for example, along the interaction structure of atraveling wave tube.

As a feature of the invention the cathode of the present electron gun isprovided with an extension which projects forward of the emissiveportion whereby an extended low velocity drift region is simply achievedwith a minimum of perturbing forces in this critical region. An electronbeam of high power with low noise is thus achieved.

The invention is described more specifically with reference to the soleaccompanying drawing which is a longitudinal cross section view of anembodiment of an electron gun structure according to the invention asemployed in a traveling wave tube.

As illustrated in the figure, the electron gun structure of theinvention includes a cathode 10, a cathode end hat electrode 11, ananode 12, a first accelerating electrode 13, a second acceleratingelectrode 14, and a drift tube 15.

The electron gun is illustrated as employed in a travel ing wave tubewhich includes, in addition to the gun structure, a slow Wave structure,illustrated as a helix 16, an input coupler 17 for introducing R.F.energy to the helix, an output coupler 18 for extracting R.F. energyfrom the helix, and a collector electrode 19 for capturing the electronsof the projected beam. Thus the electron gun and the collector electrodedefine a path through the interaction structure for an electron beam. (Acoaxial line 20 is provided for transmitting R.F. energy to the inputcoupler 17. Similarly, a coaxial line 21 is provided for transmittingR.F. energy from the output coupler 18.) The electron gun structure, theslow wave structure and the collector electrode are enclosed in anevacuated envelope 22 which may be formed of a material such as glass.

For proper operation of the traveling wave tube and of the electron gunof the invention, a magnetic field directed along the axis of the tubeis required. For this purpose an electromagnet 23 is illustrated, itbeing understood that other well-known types of magnet structures can beused. The axial magnetic field provides a force for focusing theextracted electrons into a hollow cylindrical beam 9 along the tubeaxis.

A power supply, illustrated as a battery 24, provides appropriateoperating potentials for the various elements of the electron gun andfor the collector electrode, it being understood that each terminal ofthe battery 24 is connected to a similarly numbered terminal of thetube. Typical operating voltages are shown. Further features and detailsof operation of the traveling wave tube are well-known and will not berepeated herein, the present invention being directed to an improvedelectron gun.

The electron gun of the invention will now be discussed in greaterdetail. The cathode 10, as illustrated, is circular in cross section toits longitudinal axis. The cathode 10 includes a frusto-conical portion25, that is, portion 25 is tapered in the direction of projection of theelectron beam. Along this frusto-conical portion an emissive surface 26,formed of any suitable well-known emissive material, is provided. (Aheater element 27 is supplied with a current through a pair of terminals28 to maintain the emissive surface 26 at the appropriate operatingtemperature.

The anode 12 is provided for extracting electrons from the emissivesurface 26. The ring shaped anode 12 is positioned coaxial of thecathode and it is formed with an internal bore surface which is at leastcoextensive with the emissive surface 26. The internal bore of the anode12 is preferably formed with a taper. (It is noted that an anode formedof a series of similar disk shaped electrodes to which successivelyhigher potentials are applied will provide an electric field similar tothat provided by the illustrated ring shaped anode.)

The anode 12 is operated at an electron extracting potential withrespect to the cathode 10 to thus form an electron extraction regionbetween the emissive surface 26 and the anode 12. In the absence of amagnetic field, the extracted electrons would strike the anode 12.However, the electron gun structure is immersed in the longitudinal oraxial magnetic field of the magnet 23. By the combined influence of theelectric and magnetic fields, the region between the cathode 10 and theanode 12 is constituted a magnetron space in which the extractedelectrons describe circumferentially directed paths about the cathode.This type of an electron extracting structure provides a relativelylarge emissive surface as compared, for example, to an annularly shapedcathode of similar diameter. Thus large electron currents can beextracted at relatively low emission densities.

It has been found that beam noise is reduced by providing an extendedlow velocity drift region immediately following the region of electronemission. To provide this low velocity drift region the cathode 10 isformed with an extension 30 which projects forward of the emissivesurface. The extension 30 maintains the drift region between theextraction region and the first accelerating electrode 13 at a lowerpotential than would obtain in the absence of the extension 30. Theextension 30 also aids in establishing a smooth acceleration within thelow velocity region.

-Additionally the first accelerating electrode 13 is operated at arelatively low potential, typically below the potential of the anode 12.Thus under the combined influence of the electric and magnetic fields,the extracted electrons move in spiral paths of decreasing radii towardthe first accelerating electrode 13, it being noted that the taper ofthe emissive portion of the cathode aids in imparting axial velocity tothe electrons. The cathode taper also is an aid to avoiding space chargeinstability, the nature of which is not well understood, which canotherwise occur in the electron emission and extraction region underhigh magnetic field conditions. To this end, it is also desirable toavoid large length-to-diameter ratios for the emissive surface.

After the electrons traverse the low velocity drift region between theextraction region and the first accelerating electrode, they come withinthe influence of the electric fields of the second acceleratingelectrode 14 and the drift tube 15 by which the electrons constitutingthe beam are accelerated and projected through the helix to thecollector 19.

It is noted that the extension is preferably formed with a conicallytapered end portion 31. It has been found that this tapered end portionmaterially aids in minimizing disturbances to the electron beam in itstransition from the low velocity drift region to the region of morerapid acceleration by the electric fields of the second acceleratingelectrode 14 and the drift tube 15. (A discussion of the design andoperation of accelerating regions, such as between the firstaccelerating electrode 13 and the drift tube 15, for minimum excitationof noise may be found in the aforementioned publication Noise inElectron Devices) What has thus been described is an improved electrongun of the magnetron type which provides a large emission surface forhigh power capability and which provides an extended low velocity driftregion for minimization of noise.

Thus, while the principles of the invention have now been made clear inan illustrative embodiment there will be obvious to those skilled in theart many modifications in structure, arrangement, proportions, theelements, materials, and components used in the practice of theinvention which are specifically adapted for specific environments andoperating requirements, without departing from those principles. Theappended claims are therefore intended to cover and embrace any suchmodifications, within the limits only of the true spirit and scope ofthe invention.

What is claimed is:

1. A high perveance electron gun for producing and projecting a hollowbeam of electrons in a forward direction along a predetermined path,comprising: a cathode of substantially circular cross section andincluding a first portion of decreasing cross section area in thedirection of said path, said cathode having an emissive surface along atleast a part of said first portion, said cathode having a second portionwhich is substantially non-emissive extending forward of said firstportion; an anode in spaced coaxial relationship to said cathode andhaving an internal surface at least coextensive with said emissivesurface of said cathode, the internal diameter of said anode decreasingin the direction of said path; means providing a potential differencebetween said anode and said cathode for extracting electrons from saidemissive surface; means for providing a magnet field about said cathodein the direction of said path for causing said extracted electrons todescribe circumferentially directed paths around said cathode; and meanspositioned along said path forward of said anode for accelerating saidextracted electrons along said path.

2. A low noise electron gun for projecting a beam of electrons in aforward direction along a predetermined path comprising: a cathodehaving at least a portion thereof tapered in the direction of said path,said cathode having an emissive surface comprising at least a part ofsaid tapered portion, a portion of said cathode extending beyond saidemissive surface in said forward direction; means for providing amagnetic field about said cathode in the direction of said path; ananode surrounding a portion of said cathode, the forward edge of saidanode being substantially aligned with the forward edge of said emissivesurface; means for establishing a potential difference between saidanode and said cathode for extracting electrons from said emissivesurface; and at least one accelerating electrode positioned along saidpath forward of said anode for accelerating said electrons along saidpath.

3. A low noise electron gun for projecting a beam of electrons in aforward direction along a predetermined path, comprising: a cathodeincluding a frusto-conical portion, said cathode having an emissivesurface extending along at least a part of said frusto-conical portion;means for providing a magnetic field about said cathode in the directionof said path; an anode disposed coaxially about said cathode, saidcathode having a substantially non-emissive portion extending forward ofsaid emissive surface and forward of the forward edge of said anode;means for establishing a potential difference between said anode andsaid cathode for extracting electrons from said emissive surface; and atleast one accelerating electrode positioned along said path forward ofsaid anode.

4. A low noise high perveance electron gun for projecting a beam ofelectrons along a predetermined path, comprising: a cathode including aportion of decreasing cross section area in the direction of said path,said cahode having an emissive surface along at least a part of saidportion; an anode spaced apart from and in electron extractingrelationship to said emissive surface of said cathode, said cathodefurther including a substantially non-emissive extension forward of saidemissive surface; means for establishing a potential difference betweensaid anode and said cathode for extracting electrons from said emissivesurface; means providing a magnetic field about said cathodesubstantially parallel to said path for causing said extracted electronsto describe circumferentially directed paths around said cathode; andelectron accelerating means for directing extracted electrons in theregion of said cathode along said path.

5. An electron gun for producing and projecting a beam of electrons,comprising: an elongated cathode of substantially circular crosssection, said cathode having an emissive surface along a first portionthereof; an anode in spaced coaxial relationship to said cathode andhaving an internal surface at least coextensive with said emissivesurface of said cathode; means providing an electric field between saidcathode and said anode for extracting electrons from said emissivesurface; means for immersing said cathode in a magnetic fieldsubstantially parallel to the longitudinal axis of said cathode wherebythe combined influence of said electric and magnetic fields causes saidextracted electrons to describe circumferentially directed paths aroundsaid cathode; and electron accelerating means for projecting saidelectrons from said circumferentially directed paths into a beam havinga longitudinal axis substantially coincident with said longitudinal axisof said cathode, said cathode having a substantially non-emissive secondportion projecting in the direction of said beam from said emissivefirst portion for decreasing the electron accelerating force in theregion adjacent said second portion of said cathode.

6. A low noise high perveance electron gun for projecting a beam ofelectrons along a predetermined path, comprising: a cathode including aportion of decreasing cross section area in the direction of said path,said cathode having an emissive surface along at least a part of saidportion; an anode spaced apart from and in electron extractingrelationship to said emissive surface of said cathode, said cathodefurther including a substantially non-emissive extension forward of saidemissive surface; means for establishing a potential difference betweensaid anode and said cathode for extracting electrons from said emissivesurface; means providing a magnetic field about said cathodesubstantially parallel to said path for causing said extracted electronsto describe circumferentially directed paths around said cathode; aplurality of accelerating electrodes positioned along said path forwardof said cathode; means providing a potential difference between saidcathode and each of said accelerating electrodes, the potentialdifference between said cathode and the accelerating electrode nearestsaid cathode being less than said potential difference between saidanode and said cathode.

7. A low noise high perveance electron gun for projecting a beam ofelectrons along a predetermined path, comprising: a cathode including aportion of decreasing cross section area in the direction of said path,said cathode having an emissive surface along at least a part of saidportion; an anode spaced apart from and in electron extractingrelationship to said emissive surface of said cathode, said cathodefurther including a substantially non-emissive extension forward of saidemissive surface, said extension being formed with a conically taperedend portion; means for establishing a potential difference between saidanode and said cathode for extracting electrons from said emissivesurface; means providing a magnetic field about said cathodesubstantially parallel to said path for causing said extracted electronsto describe circumferentially directed paths around said cathode; andelectron accelerating means for directing extracted electrons in theregion of said cathode along said path.

No references cited.

DAVID G. REDINBAUGH, Primary Examiner.

2. A LOW NOISE ELECTRON GUN FOR PROJECTING A BEAM OF ELECTRONS IN AFORWARD DIRECTION ALONG A PREDETERMINED PATH COMPRISING: A CATHODEHAVING AT LEAST A PORTION THEREOF TAPERED IN THE DIRECTION OF SAID PATH,SAID CATHODE HAVING AN EMISSIVE SURFACE COMPRISING AT LEAST A PART OFSAID TAPERED PORTION, A PORTION OF SAID CATHODE EXTENDING BEYOND SAIDEMISSIVE SURFACE IN SAID FORWARD DIRECTION; MEANS FOR PROVIDING AMAGNETIC FIELD ABOUT SAID CATHODE IN THE DIRECTION OF SAID PATH; ANANODE SURROUNDING A PORTION OF SAID CATHODE, THE FORWARD EDGE OF SAIDANODE BEING SUBSTANTIALLY ALIGNED WITH THE FORWARD EDGE OF SAID EMISSIVESURFACE; MEANS FOR ESTABLISHING A POTENTIAL DIFFERENCE BETWEEN SAIDANODE AND SAID CATHODE FOR EXTRACTING ELECTRONS FROM SAID EMISSIVESURFACE; AND AT LEAST ONE ACCELERATING ELECTRODE POSITIONED ALONG SAIDPATH FORWARD OF SAID ANODE FOR ACCELERATING SAID ELECTRONS ALONG SAIDPATH.